Oecologia (Berlin) (1986) 70:234-237 Oecologia 9 Springer-Verlag1986 Mistletoes: a hypothesis concerning morphological and chemical avoidance of herbivory J.R. Ehleringer 1, I. Ullmann 2, O.L. Lange 2, G.D. Farquhar 3, I.R. Cowan 3, E.-D. Sehulze 4, and H. Ziegler 5 1 Department of Biology, University of Utah, Salt Lake City, UT 84112, USA 2 Lehrstuhl ffir Botanik II der Universit~it, Mittlerer Dallenbergweg 64, D-8700 Wiirzburg, Federal Republic of Germany 3 Department of Environmental Biology, Research School of Biological Sciences, Australian National University, Canberra, ACT Australia 2601 4 Lehrstuhl fiir Pflanzen6kologie, Universit~it Bayreuth, D-8580 Bayreuth, Federal Republic of Germany 5 Botanisches Institut, der Technischen Universit/it, D-8000 Miinchen, Federal Republic of Germany Summary. Leaves from many misletoe species in Australia strongly resemble those of their hosts. This cryptic mimicry has been hypothesized to be a means of reducing the likeli- hood of mistletoe herbivory by vertebrates. Leaf Kjeldahl nitrogen contents (a measure of reduced nitrogen and thus amines, amino acids and protein levels) of mistletoes and their hosts were measured on 48 mimetic and nonmimetic host-parasite pairs to evaluate hypotheses concerning the significance of crypsis versus noncrypsis. The hypothesis that mistletoes mimicking host leaves should have higher leaf nitrogen levels than their hosts is supported; they may be gaining a selective advantage through crypsis (reduced herbivory). The second hypothesis that mistletoes which do not mimic their hosts should have lower leaf nitrogen levels than their hosts is also supported; they may be gain- ing a selective advantage through noncrypsis (reduced her- bivory resulting from visual advertisement of their reduced nutritional status). Key words: Mimicry - Nitrogen Herbivory - Mistletoe - Australia Early botanical explorations of the Australian flora have noted a strong similarity in appearance between the leaves of a number of mistletoe species and their hosts (Drum- mond 1840; Hemsley 1896; Moore 1899). Not all mistletoes mimic their hosts, but rather there is one subset of mistletoe species that strongly resembles host leaf shape and another subset of mistletoe whose leaves clearly are different in size and shape from those of their hosts. The striking resem- blance of host and parasite leaves was best described by Barlow and Wiens (1977), who proposed that it was an example of protective cryptic mimicry. The authors indi- cated that cryptic mimicry in mistletoes had evolved as a means of avoiding predation by herbivores. Furthermore, they suggested that cryptic mimicry by mistletoes may have been to avoid vertebrate herbivory, since vertebrates more often rely on visual cues as opposed to insects which more frequently use olfactory cues. Underlying this hypothesis is the notion that host-re- sembling mistletoes are a more favorable food resource for Offprint requests to." J.R. Ehleringer some unknown reason. Barlow and Wiens (1977) suggested that perhaps mistletoes were preferrable because of higher water contents and that by visually blending in with the background of their hosts, the probability of being found and predated was reduced. However, no data on leaf quality (water content, nutritional, antiherbivore defense, etc.) were available to evaluate their hypothesis. Mattson (1980) reviewed a number of studies of herbi- vory, which indicated that leaf tissue protein levels affect feeding patterns in animals (especially vertebrates). There was a preference toward feeding on leaf tissues with higher levels of protein. Animals may prefer different food re- sources for a variety of reasons (water content, vitamins, carbohydrates, etc.), and separating out the influences of these factors from protein levels may be difficult. Although higher protein contents may be only one component, it is a major component and can form the basis of testable hypotheses concerning the evolutionary bases of cryptic mimicry in mistletoes. As a refinement of the Barlow-Wiens hypothesis for cryptic mimicry in Australian mistletoes, we suggest that leaf mimicry in mistletoes evolved as a means of reducing predation because host-mimicking mistletoes contain higher protein levels than their hosts. However, we realize that this need not be the only reason for mimicry. It is also possible that under some circumstances, hosts may possess higher concentrations of antiherbivore compounds and that mistletoes could derive some protection in these cases by mimicking their hosts, irrespective of their relative levels of leaf protein. On the other hand, there may be some advantage to a mistletoe to appear different from its host, if it turned out that the mistletoe was of lower nutritional quality than was its host. If the mistletoe had on average a lower leaf protein content than did its host, it would be advantageous to alert potential predators to this difference by contrasting in leaf size or shape. The contrasting morphologies could be a visual advertisement of the mistletoe's lower nutritional status. We have two hypotheses concerning cryptic mimicry, nonmimicry and protein levels of Australian mistletoes and their hosts: 1. Mistletoes that mimic their hosts will have higher lev- els of leaf protein than their hosts (cryptic mimicry). They